Soil capping is the forming of a hard surface crust in the top 1 to 10 mm of bare cultivated soils.
Caused by heavy rainfall on exposed bare land, the surface structure of the soil breaks down under the continual pounding of the water droplets. This is then hardened by sun and drying winds, to give a thin crusty surface that is sometimes cracked.
Most at risk are soils with a fine sandy or silty structure, though clay soils that have been finely cultivated can also be very prone to surface capping. Soils that contain little or no organic matter in their composition are most vulnerable.
A capped surface prevents the soil beneath absorbing significant amounts of air and water that is required by the growing vegetable plants, this results in much slower and weaker growth, most often causing the plant to run to seed prematurely.
Germinating seeds find it near impossible to break through the capped soil surface, and soon run out of strength, resulting in the loss of the crop.
Solving a capped soil surface can be as simple, as regular hoeing around established plants, or between rows of seedling, to loosen up the surface. On bare soils, a mulch of garden compost can be used to help prevent surface capping.
But by far the best solution to soil capping is adding organic matter to the soil. This will help the soil absorb the heavy rainfall, improve the structure of the soil and help with soil fertility.